1. Field of the Invention
The present invention relates to a method and apparatus for forming an electrolytic water and an apparatus for washing semiconductor wafers using an electrolytic water-forming apparatus.
2. Description of the Related Art
It was customary in the past to use fluorine-based solvents such as freon in the manufacture of semiconductor devices. However, since the fluorine-based solvents give detrimental effects to living environments, water such as a pure water and ultra-pure water has come to be used as a safe solvent in place of the fluorine-based solvents.
The pure water is substantially free from impurities such as ions, fine particles, microorganisms and organic substances and exhibits such a high resistivity as 5 to 18 Mxcexa9xc2x7cm. The ultra-pure water, which is prepared by using an ultra-pure water manufacturing apparatus, is prepared by removing with a high efficiency the substances which are suspended or dissolved in water, and has a purity higher than that of the pure water.
In terms of the electrical conductivity, the pure water has a conductivity xcfx81 lower than 10 xcexcScm. On the other hand, the ultra-pure water has a conductivity xcfx81 lower than 0.055 xcexcScm. If the pure water or the ultra-pure water is electrolyzed, produced are an anodic water containing substances exhibiting a strong oxidizing power such as oxygen and ozone and a cathodic water exhibiting a strong reducing power.
In the manufacture of a semiconductor device, a surface of a semiconductor substrate is washed with, for example, an anodic water or a cathodic water.
In the conventional method of manufacturing a semiconductor device, a surface of a silicon semiconductor substrate is etched in a small amount to remove particles, and a pH value of the etching solution is controlled to remove metallic contaminants, followed by washing the substrate with an ultra-pure water until the resistivity of the ultra-pure water is recovered to about 18 Mxcexa9xc2x7cm. The oxidizing/reducing properties of the washing water are affected by the substances contained therein. It should be noted that oxygen is dissolved in the pure or ultra-pure water. However, if a silicon semiconductor substrate is washed wish such a pure water, the substrate is partially oxidized by the dissolved oxygen. To overcome the difficulty, it was customary to diminish the amount of oxygen dissolved in the washing water.
The diameter of a semiconductor wafer is expected to continue to be enlarged. In accordance with enlargement in the diameter of the semiconductor wafer, it is required to solve various problems including, for example, an increased running cost, an increased amount of a chemical solution and ultra-pure water used for washing the surface of a semiconductor substrate, and an increased amount of waste liquids which must be disposed of. In general, the chemical solution used for the washing of the semiconductor substrate is harmful to the human body, making it necessary to handle the chemical solution very carefully. It is also necessary to dispose of the waste liquid of the chemical solution appropriately so as not to give detrimental effects to the living environment.
In the conventional method of electrolyzing an ultra-pure water, a halogen-containing material such as NaCl is used as a supporting electrolyte. As a result, the halogen is contained in the electrolytic water. It follows that the halogen remains on the semiconductor substrate after washing with the electrolytic water. What should be noted is that the remaining halogen causes problems such as deterioration of an oxide film.
The present invention, which has been achieved in an attempt to solve the above-noted problems inherent in the prior art, is intended to provide a method and apparatus for forming an electrolytic water from a pure water or an ultra-pure water, and to provide an apparatus for washing a semiconductor substrate by using an electrolytic water-forming apparatus. The method and apparatus of the present invention make it possible to wash a semiconductor substrate without doing damage to the substrate, to decrease the running cost in the washing step of the semiconductor substrate, to make the waste water harmless, and to improve the safety in the working environment.
According to an aspect of the present invention, there is provided a method of forming an electrolytic water, comprising the steps of:
preparing a saturated solution by adding at least one solid supporting electrolyte selected from the group consisting of oxalic acid, ammonium oxalate, ammonium formate, ammonium bicarbonate, and ammonium tartrate to a pure water or an ultra-pure water; and
subjecting the saturated solution to electrolysis to obtain an anodic water and a cathodic water.
According to another aspect of the present invention, there is provided an apparatus for forming an electrolytic water, comprising:
a dissolving vessel provided with a layer of at least one solid supporting electrolyte selected from the group consisting of oxalic acid, ammonium oxalate, ammonium formate, ammonium bicarbonate, and ammonium tartrate, a pure water or an ultra-pure water being added to the solid supporting electrolyte to prepare a solution saturated with the supporting electrolyte;
a mixing vessel in which the saturated solution of the supporting electrolyte, which is prepared in the dissolving vessel, is further mixed with a pure water or an ultra-pure water for diluting the saturated solution;
an electrolytic vessel in which the diluted solution is electrolyzed, the electrolytic vessel having an anode chamber and a cathode chamber in which an anodic water and a cathodic water are generated, respectively, as a result of the electrolysis;
a first pipe line for supplying a first pure water or ultra-pure water into the mixing vessel together with the saturated solution withdrawn out of the dissolving vessel; and
a second pipe line for supplying a pure water or ultra-pure water into the dissolving vessel through a lower portion of the dissolving vessel.
Further, according to still another embodiment of the present invention, there is provided an apparatus for washing a semiconductor wafer by using an electrolytic water forming apparatus, the washing apparatus comprising:
a dissolving vessel provided with a layer of at least one solid supporting electrolyte selected from the group consisting of oxalic acid, ammonium oxalate, ammonium formate, ammonium bicarbonate, and ammonium tartrate, a pure water or an ultra-pure water being added to the solid supporting electrolyte to prepare a solution saturated with the supporting electrolyte;
a mixing vessel in which the saturated solution of the supporting electrolyte, which is prepared in the dissolving vessel, is further mixed with a pure water or an ultra-pure water for diluting the saturated solution;
an electrolytic vessel in which the diluted solution is electrolyzed, the electrolytic vessel having an anode chamber and a cathode chamber in which an anodic water and a cathodic water are generated, respectively, as a result of the electrolysis;
a first pipe line for supplying a first pure water or ultra-pure water into the mixing vessel together with the saturated solution withdrawn out of the dissolving vessel;
a second pipe line for supplying a pure water or ultra-pure water into the dissolving vessel through a lower portion of the dissolving vessel;
a washing vessel having semiconductor wafers housed therein; and
an electrolytic water supply pipe line connecting the electrolytic vessel and the washing vessel.
In the present invention, a halogen-containing substance is not used as a supporting electrolyte in forming an electrolytic water. Naturally, use of the particular electrolytic water for washing a silicon semiconductor substrate permits overcoming the problem inherent in the prior art, i.e., the problem that the halogen remains on the semiconductor substrate after the washing step so as to bring about deterioration of the oxide film formed on the substrate.